Just listened to last week's spoiler cast after beating Portal 2 this afternoon, and I want to thank Garnet for at least briefly trying to differentiate between story and exposition. I really felt like the game began to drag in terms of pacing after GLaDOS was turned into a Potato.
Contrary to Jeff, I found it incredibly anti-climactic to have the plot progress so rapidly and then be forced back down into another series of rigidly designed Portal puzzles. Like Garnet, I found Wheatley's turn from impotent lab dweller, to wrathful overlord to be incredibly trite and predictable. This is basically the same villain archetype of the bad guy in The Incredibles, right?
Incidentally, I think a lot of the problems I have with this game's presentation are the same ones I have with Bioshock. Although I like how thoughtful the back story to Portal 2 was (the moon rocks, etc), I find the conceit of presenting everything via loud speaker in the world to be very lazy and a creative crutch.
I'm sick of this structure in games of recent years where all plot development is thrown out the window, and instead we get a running radio play, or basically a podcast in the background of what's actually happening.
This is sort of like in Mass Effect games, if Bioware took the audio encyclopedia found in the game's start menu, and only that the entire game's plot. Structurally it's a complete cop out to the challenge of designing story around the experience of the player.
Again, its the same in Bioshock. All the actual "drama" occurs in expository files, or recorded diaries---oh and then occasionally you get to see a character murdered from behind a convenient giant glass window. I find this whole notion of "story" to be incredibly uninvolving, and really sort of insulting to gaming audiences.
I think the irony of people who complain about cut-scenes in games, is that when you don't have the legitimate plot development they provide, your character ends up becoming a passive vessel to the game's world. You're not only not engaging with the plot as a character, you're not even in a plot. All the so-called story is a distant thing involving other players.
I of course loved Portal's clever dry humor though, and generally found it to be a fun experience. I just found much of the last half or so to be quite painfully lacking in any significant plot development.